Pole tubes for electromagnets, in particular for magnetic valves in automatic transmissions in automobiles, are known in one-part as well as multi-part execution. One-part pole tubes, such as described in DE 10 2006 015 233 B4, are advantageous concerning the possibility of a low-friction guiding of the magnetic armature and a small radial air gap to the inner lateral surface of the pole tube. This is enabled by the uninterrupted or gap-free design of the pole tube, whereby the inner lateral surface can be utilized directly as a guiding surface for the armature. For suppressing a magnetic short circuit via the pole tube, said pole tube is usually machined thinner in one place.
For all multi-part pole tubes, the magnetic separation of the pole tube is ensured by a nonmagnetic material. The separation of the pole tube can be integrated in the pole tube by a separate part in the form of a pressed sleeve or, such as described in DE 10 2006 015 070 A1, by a welded nonmagnetic ring. For the multi-part execution of the pole tube, it is advantageous to avoid the magnetic short circuit via the pole tube. In principle, it should thereby be able to attained higher magnetic forces.
FIG. 4 shows a connection, known from DE 10 2013 226 619 A1, of two pole tube halves by means of a nonmagnetic ring and casting with an outer plastic layer serving as a winding carrier for a coil winding. The pole tube 110 consists of a pole core 112, a magnetic tube 114 as well as an intermediate nonmagnetic ring 116 as pole tube parts. In the manufacture, the pole tube parts are slipped on a centering arbor and thus arranged in mutually concentric relationship. Thereafter, an outer lateral surface 120 of the pole tube parts is insert molded, for example with a plastic. FIG. 4 shows the pole tube 110 with the insert-molding layer 122 applied to the outer lateral surface 120.
Due to the diameter tolerances of the pole tube parts, an offset is inevitable between the respectively mutually adjacent pole tube parts at the armature running surface 124 formed at the interior of the pole tube 110. No offset-free inner surface can thus be attained for the pole tube. The radial air gap between armature running surface 124 and a magnetic armature (not shown in FIG. 4) displaceably arranged in the pole tube 110 can therefore not be adjusted arbitrarily small. That is, a non-clamping guiding of the magnetic armature directly or by means of an additional PTFE foil in the pole tube 110 can be ensured only by a correspondingly larger dimensioned radial guiding gap at the expenses of the magnetic force to be attained. To avoid this, the mounted pole tube could be reworked. This is not desirable from an economic point of view. A further disadvantage of a multi-part pole tube with intermediate ring 116 are the associated production and mounting expenses.